Subpixel movement means that your game is actually being rendered in 480x320 and not 84x56, because even though the camera size in your game engine is set to 84x56, when you scale the window, things aren't drawn snapped to the nearest integer position; so when your player's y-position is 4.85 for example, it's drawn at y=4.85 and not 5, so it's slightly higher than if the resolution was actually 84x56.
In other words, when you scale down a screenshot of the game to 84x56, it appears to be losing quality - this should not be the case.
(The image on the right has a resolution of 84x56 pixels.) According to the rules, the resolution should be 84 x 48 pixels, without subpixel movement. I don't know how to change this in Unity.